So while I was working on my upcoming minigame map, I have noticed that some of the command blocks I used would not load up their GUI if opened by one of the guests in a LAN world. The commands in them wouldn't show up, all the buttons were locked.
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I just did, and it doesn't seem to happen. So I guess this a corrupted world kind of issue. Are those valid?
OK, I did a bit of testing, and this is what I found; Fresh worlds probably don't have this issue, but any command blocks, old or new, in worlds from previous snapshots have this issue.

Not an issue, snapshots will corrupt worlds...
There is no upgrade path for issues introduced in one snapshot but no longer happening in another snapshot.
I know snapshots corrupt worlds, but I don't expect them to corrupt worlds that were also from snapshots, especially if the worlds are only one snapshot old, especially since the new snapshot doesn't change anything related to command blocks...
I expect this to be fixed in the final 1.9 release.
If you can reproduce the issue with a world generated on a release version and only opened in the latest snapshot it is valid, otherwise, as @unknown explained, no.
Well, I just realized that this bug is for guests in a LAN world, so reopen please.
Can somebody reopen this?

No, but linked to MC-88632
OK, thanks, that's better. Please remove the link to MC-87872, as it has nothing to do with the issue
Can you recreate that issue in a fresh 15w42a world with new placed command blocks ?